This invention relates to a navigational system for determining the bearing and distance of one object with respect to another, the two objects being above ground in the earth's atmosphere or in outer space. It more particularly relates to a radio navigation system using multiple fixed frequencies.
The known related art systems do not use a fixed antenna array to send out unmodulated radio signals. They either mechanically rotate a directional antenna at the central station or electronically simulate such a rotating antenna. Further, most known systems modulate the radio beacon as it rotates.
U.S. Pat. No. 2,616,076 to Lyman et al. teaches rotating a directional antenna 51 at the reference station, and determining the azimuth (bearing) of the mobile station from the phase of a component of a carrier signal received at the receiver of the mobile station (Column 4, lines 24-56). The Lyman et al. patent also teaches using the round-trip travel time of the radio waves between the mobile station and the reference station to determine the distance between the two stations (Column 5, lines 17-60).
U.S. Pat. No. 3,648,285 to Sanders teaches using an interrogation pulse and the two-way radiation time to determine the distance of the mobile station (an airborne aircraft) from the fixed station (a ground station) (Column 2, lines 42-45). The Sanders patent teaches using a ground station having a rapid elevation frequency scan and a somewhat lower azimuth phase scan of a planar array. Elevation angle data is air-derived from the frequency of the pencil beam energy as it passes across the aircraft. Azimuth position of the ground antenna pencil beam is separately supplied through an independent modulation which is correspondingly decoded in the air.
U.S. Pat. No. 3,721,950 to Jorgensen et al. teaches using a navigation beacon having a pair of responsive devices rotating at different angular velocities. In operation, an observer at a particular bearing relative to the beacon receives two distinct responses from a pair of reflectors or transponders. Bearing is determined from the time interval between the two distinct responses. Range is determined from the interval between interrogation and response.
U.S. Pat. No. 4,017,860 to Earp teaches using a fixed planar matrix of antennas with programmed excitation to produce a moving pencil beam of radiation. This produces an artificial Doppler signal component as observed at a remote receiving station. Navigational information is derived from the signal using the principle that the Doppler shift in frequency is proportional to the cosine of the angle which a radio receiver of the system subtends with respect to the line of movement of the source.
Known existing devices transmit either a rotating and/or modulating radio beacon. Rotating the radio beacon is disadvantageous because the navigational signal is transmitted to only a small volume of space at any one time. Modulating the radio beacon is disadvantageous because the time consumed in modulating and demodulating limits the repeat transmission rate of the navigational signal.
Applicant overcomes the above-noted and other drawbacks of the known related art by providing a method and apparatus for a radio navigation system using multiple fixed frequencies. A system according to the present invention features the capability of providing both bearing and distance information. The present invention eliminates the undesirable feature common to known existing devices wherein the radio beacon transmitted by the central station is rotated and/or modulated. Applicant believes that the preferred embodiment disclosed herein is the first radio navigation system which: (1) uses multiple fixed frequencies; (2) provides both bearing and distance information; and (3) does not require a rotating or modulating radio beacon from the central station. Thus, the present invention allows navigational signals to be transmitted over the entire area of interest simultaneously, with a rapid repeat transmission rate.
A radio navigation system according to the present invention uses multiple fixed frequencies for calculating the bearing and distance of a local station from a central station. According to one aspect of the present invention, the central station has a receiver and an array of fixed, directional antennas all transmitting signals at different frequencies. According to another aspect of the present invention, the local station has a transmitter for transmitting a query signal to the central station, a plurality of local receivers to receive the signals from the array, and a stored-program processor.
In one embodiment of the present invention, each local receiver operates at a fixed frequency identical to the frequency at which its respective central transmitter and central antenna is operating. A stored-program processor measures the elapsed time between transmission of the query signal from the local station and receipt of a navigational signal from the central transmitters. Using this data, the processor calculates the distance between the local station and the central station. The stored-program processor also compares the relative signal strengths of the signals received by the local station from the various antennas in the antenna array of the central station. Using this data, the processor calculates the bearing of the local station from the central station.
The above-noted and other aspects of the present invention will become more apparent from a description of the preferred embodiment when read in conjunction with the drawings.